Consumers of bulk quantities of soluble solid materials often change to bulk solutions when their processes permit this, because of the ease of unloading and feeding solutions to processing equipment. Depending on the solid's solubility in the solvent used, which is most often water, the solvent weight can be 50 to 75 percent of the total weight of the material. Since transportation costs vary directly with the weight of the material transported, partial or complete removal of the solvent greatly reduces the costs of transportation alone. However, drying to remove the water of solution is also expensive.
Bulk quantities of dry soluble solid materials can be transported via tank type railroad cars and truck tank trailers and unloaded by dissolving the material out of the transport vehicle. This method of unloading is known as sparging. This is less expensive than shipping solutions.
Thus a tank car containing water soluble solids that will be used as a solution can be shipped dry but unloaded at its destination by a sparging process. We refer to U.S. Pat. No. 3,759,279 to Smith, Jr. which unloads dry sugar from a truck tank trailer by dissolving it out of the trailer with heated water. Such tank trailers are fitted with multiple covered openings distributed along the length of the top of the tank for loading dry material into the trailer.
The sparging process entails recirculating that amount of water from a storage tank to the tank car that is needed to dissolve the solids and dilute the resultant solution to the desired concentration. The water for dissolving the solids is pumped into the bottom of the tank car through a distribution header, and flows to the upper outlet of the tank car from which it exits the car and flows to the storage tank. The water is recirculated from the storage tank, from which it again returns to the tank car, until the solids are dissolved. The rate of dissolution generally can be increased if the solution is heated en route to the tank car. If the tank car can be permitted to become slightly pressurized, then only one pump is required to move all of the solutions. Unloading a sparger car can take up to 24 hours. This system for unloading soluble solids has found widespread use.
FIG. 1 illustrates schematically a typical sparger unloading system for a tank car. Referring to FIG. 1, a railroad tank car 12 has an inlet line 14 leading from the bottom of a storage tank 16. The storage tank 16 is loaded with the desired level of water required to dissolve the solids being unloaded. The water at first, and later recirculated solution, is fed from the storage tank 16 through the bottom header 18 of the tank car 12, which contains the solids to be dissolved. the solution leaves the car through an outlet line 20. The tank car dome 22 is closed so the pressure from a pump 24 forces the solution out of the tank car 12 to the top of the storage tank 16. The increase of solution concentration can be monitored by checking the specific gravity of a sample of the returned solution. The tank car 12 can be inspected when the pumping has been stopped and the dome cover 22 is opened. The best indication of complete dissolution would be several successive hydrometer readings with no increase in specific gravity.
To load a tank car so that the solids are fairly distributed in the tank car, multiple domes have been used. In the prior art generally as many as five domes have been installed in a railroad tank car. The cited patent, supra, includes a sketch of a truck trailer with nine openings distributed along the length of the top. This type of car or truck is usually loaded using some type of mechanical conveyor. A specific amount of material is loaded into each opening. The conveyor must be stopped and the car moved to load through each opening to the desired load. In this fashion the car is filled safely to something less than the load limit. Particular care has to be taken not to overload the car. For this system of loading dry products, a costly conveyor system must be installed. If it is overloaded, the excess solids may have to be removed with costly hand labor.
Thus an improved method of loading tank cars with solid materials would be highly desirable.